The present invention relates generally to an apparatus and a method for controlling a water heater system and, more particularly, to a system for quantitatively controlling a water heater in accordance with a measured demand for hot water.
Any system for providing hot water in a building must be capable of providing sufficient hot water to supply the needs of the building's occupants. Traditionally, this has been accomplished using an internal thermostatic control, or "aquastat", to control the temperature of the heated water to a fixed set point high enough to meet the demand for hot water during peak periods, such as early morning and late evening hours. This practice results in substantial loss of heat by conduction and convection during non-peak periods, however, and is particularly inefficient in large recirculating hot water systems. It is therefore desirable to reduce the set point temperature of a hot water system during off-peak periods to conserve energy.
A number of supplemental control systems have been proposed for altering the operation of large, recirculating water heating systems during off-peak periods as a way of conserving energy. One such system uses a time clock to switch from a relatively high control temperature during peak use periods to a lower control temperature at off-peak periods. Other systems make use of historical data to control water heaters, as described in U.S. Pat. Nos. 4,620,667 and 4,834,284, both to Vandermeyden, and U.S. Pat. No. 5,056,712 to Enck. The systems of the Vandermeyden patents are typically operated in series with the aquastat of an associated water heater. They develop a separate control temperature based on historical usage data representing the proportion of time during a preceding period that the water heater's burner was "on". Such systems lag significantly behind the actual demand for heated water, however.
The system of the Enck patent senses hot water usage by monitoring the temperature of "cold" water in the system's water supply inlet at a point adjacent a hot water storage tank. The water within this "migration zone" is heated somewhat by the storage tank itself and thus reaches an intermediate temperature after a period of low hot water usage. The intermediate temperature is suddenly lowered, however, when additional water is introduced to replenish hot water depleted by consumers, providing an early indication of hot water use. This information is then used to determine an appropriate set back temperature range, depending on whether the system is within "an historically indicated hot water use period." The Enck system therefore uses inlet water temperature in a qualitative sense to select an operating temperature range based on historical information. Unfortunately, the use of historical information complicates the system and renders it less responsive to current usage demands.
Therefore, it is desirable in many applications to provide a system for controlling the operation of a hot water heater which is highly efficient, responsive and simple in its implementation.